Automotive Systems

Formerly Automotive Systems I

automech02.jpg (42077 bytes)

shpslogo.jpg (6992 bytes)

LegalContact Us

Thermostats

Thermostats

Automatic control of the temperature of the engine is necessary for efficient engine performance and economical operation. If the engine is allowed to operate at a low temperature, sludge buildup and excessive fuel consumption will occur. On the other hand, overheating the engine or operating it above normal temperature will result in burnt valves and faulty lubrication. The latter causes early engine failure.

The thermostat senses engine temperature and controls coolant flow through the radiator. It allows coolant to circulate freely only withinthe blockuntil the desired temperature is reached. This action shortens the warm-up period. The thermostat normally fits under the thermostat housing between the engine and the end of the upper radiator hose. The pellet-type thermostat that is used in modern pressurized cooling systems incorporates the piston and spring principle (fig. 6-11).

The thermostat consists of a valve that is operated by a piston or a steel pin that fits into a small case, containing a copper impregnated wax pellet. A spring holds the piston and valve in a normally closed position. When the thermostat is heated, the pellet expands and pushes the valve open. As the pellet and thermostat cools, spring tension overcomes pellet expansion and the valve closes.

Thermostats are designed to open at specific temperatures. This is known as thermostat rating. Normal ratings are between 180F and 195F for automotive applications and between 170F and 203F for heavy-duty applications. Thermostats will begin to open at their rated temperature and are fully open about 20F higher, For example, a thermostat with a rating of 195F starts to open at that temperature and is fully open at about 215F.

Most engines have a small coolant bypass passage that permits some coolant to circulate within the cylinder block and head when the engine is cold and the thermostat is closed. This provides equal warming of the cylinders and prevents hot spots. When the engine warms up, the bypass must close or become restricted. Otherwise, the coolant would continue to circulate within the engine and too little would return to the radiator for cooling.

The bypass passage may be an internal passage or an external bypass hose. The bypass hose connects the cylinder block or head to the water pump. There are two internal bypass systems that can be used on an engine.

One internal bypass system uses a small, spring-loaded valve located in the back of the water pump. The valve is forced open by coolant pressure from the pump when the thermostat is closed. As the thermostat opens, the coolant pressure drops within the engine and the bypass valve closes.

Another bypass system has a blocking-bypass thermostat (fig. 6-12). This thermostat operates as previously described, but it also has a secondary, or bypass, valve. When the thermostat valve is closed, the circulation to the radiator is shut off. However, when the bypass valve is open, coolant is allowed to circulate through the bypass. As the thermostat valve opens, coolant flows into the radiator and the bypass valve closes.

Some stationary engines and large trucks are equipped with shutters that supplement the action of the thermostat in providing a faster warm-up and in maintaining proper operating temperatures. When the engine coolant is below a predetermined temperature, the shutters, located in front of the radiator, remain closed and restrict the flow of air through the radiator. Then as the coolant reaches proper temperature, the shutters start to open. Two methods are used to control the shutter opening. A stationary engine uses a shutterstat (long thermostatic valve) connected to the engine cooling system with hoses or pipes that allow the coolant to circulate through the valve. The temperature of the coolant, when it reaches a predetermined temperature, causes the valve to expand extending a rod which through linkage forces the shutters open. Trucks, equipped with an air brake, use a smaller thermostatic valve that actuates an air valve. This air valve allows pressure from the air tank to enter the air cylinder attached to the shutter operating mechanism, forcing the shutters open.

Figure 6-11.—Pellet-type thermostat.

Figure 6-12.—Blocking-bypass thermostat.

Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

Copyright 2001-2004 SweetHaven Publishing Services
All rights reserved